Conventional aircraft environmental control systems incorporate an air cycle machine, also referred to as an air cycle cooling machine, for use in cooling and dehumidifying air for supply to the aircraft cabin for occupant comfort. Typically, such air cycle machines are three wheel machines comprised of a compressor, a turbine, and a fan disposed at axially spaced intervals along a common shaft, the turbine driving both the compressor and the fan. The three wheels are supported for rotation about the axis of the shaft on one or more bearing assemblies disposed about the drive shaft, such as a pair of spaced bearing assemblies, one bearing assembly disposed intermediate each pair of adjacent spaced wheels. Although the bearing assemblies may be ball bearings or the like, hydrodynamic film bearings, such as gas film foil bearings, are often utilized on state-of-the-art air cycle machines. For example, three wheel air cycle machines are disclosed in U.S. Pat. Nos. 3,428,242 and 4,507,939; and also in commonly assigned, U.S. Pat. No. 5,113,672.
The three wheel air cycle machine disclosed in commonly assigned U.S. Pat. No. 3,428,242 comprises a fan, a compressor and a cooling turbine mounted to a common shaft driven by the turbine, with the fan being disposed at one end of the shaft and the turbine and compressor being disposed in back to back relationship at the other end of the shaft with the turbine inboard of the compressor. The shaft is rotatively supported on a ball bearing assembly disposed intermediate the fan and the turbine and cooled by turbine outlet air.
In U.S. Pat. No. 4,507,939, a three wheel air cycle machine is disclosed which includes a fan, a compressor and a turbine mounted to a common shaft driven by the turbine with the fan being disposed between the compressor and the turbine which are mounted to the opposite ends of the shaft. Rotational support for the shaft is provided by a pair of gas film bearings, one disposed about the shaft intermediate the fan and the compressor and the other disposed about the shaft intermediate the fan and the turbine. Both gas film bearings are cooled and pressurized with compressed air passed from the inlet to the cooling turbine through the bearing and thence discharged into the fan inlet or outlet ducts.
The three wheel air cycle machine disclosed in commonly assigned U.S. Pat. No. 5,113,672 also incorporates a fan, a compressor and turbine mounted to a common shaft driven by the turbine and supported on a pair of hydrodynamic film foil bearings. In this air cycle machine, the compressor is centrally disposed on the shaft with the fan and the turbine disposed at opposite ends of the shaft thereby providing for axial discharge of the fan exhaust and the turbine exhaust. One of the hydrodynamic film foil bearings supporting the shaft is disposed between the compressor and the turbine and the other between the compressor and the fan. A bearing cooling circuit is provided whereby compressed air from the turbine inlet duct is passed first through one bearing and thence the other bearing, to both cool and pressurize the hydrodynamic film foil bearings, and thereafter discharged into the compressor inlet duct.
On aircraft powered by turbine engines, the air to be conditioned in the air cycle machine is typically compressed air bled from one or more of the compressor stages of the turbine engine. In conventional systems, this bleed air is passed through the air cycle machine compressor wherein it is further compressed, thence passed through a condensing heat exchanger to cool the compressed air sufficiently to condense moisture therefrom thereby dehumidifying the air before expanding the dehumidified compressed air in the turbine of the air cycle machine to both extract energy from the compressed air so as to drive the shaft and also to cool the expanded turbine exhaust air for use as the cooling fluid in the aforementioned condensing heat exchanger before it is supplied to the cabin as conditioned cooling air. The compressed bleed air being supplied to the compressor of the air cycle machine is typically precooled by passing it through a precooling heat exchanger in heat exchange relationship with ambient air drawn through the precooling heat exchanger by the fan of the air cycle machine.
Although conventional three wheel air cycle machines are satisfactory for use in such conventional environmental control systems, such three wheel air cycle machines are not suitable for use in the improved aircraft environmental control system disclosed in commonly assigned U.S. Pat. No. 5,086,622, wherein the expanded and cooled turbine exhaust air which has passed as the cooling fluid in heat exchange relationship with the compressed air passing through the condensing heat exchanger is, after discharge from the cooling fluid path of the condensing heat exchanger, expanded in a second turbine to cool the expanded air to a desired temperature for supply to the cabin as conditioned cooling air and to extract further energy therefrom. To be suitable for use in this improved aircraft air conditioning system, an air cycle machine must have four wheels, i.e. two turbines, a compressor and a fan.